In general, a hybrid electric vehicle (HEV) refers to a vehicle that uses two power sources including an engine and an electric motor. An HEV has excellent fuel efficiency and engine performance compared with a vehicle including only an internal combustion engine and is also advantageous for lowering emissions, and thus, has been actively developed recently.
Such a hybrid vehicle travels in two modes according to a powertrain used to drive the vehicle. One of the modes is an electric vehicle (EV) mode in which the vehicle travels using only an electric motor and the other one of the modes is a hybrid electric vehicle (HEV) mode for operating both an electric motor and an engine to acquire power. A hybrid vehicle switches between the two modes according to driving conditions.
It is advantageous that such an EV mode does not consume fuel during driving but the EV mode cannot satisfy driver requested power that exceeds maximum motor output. In an HEV mode, a vehicle uses an engine as a main power source and, in this case, travels while a motor is being recharged or discharged as necessary. Accordingly, the HEV mode can satisfy high driver requested power and consumes fuel, unlike the EV mode.
As a result, fuel efficiency and drivability of a corresponding vehicle are determined depending on switching control between the aforementioned two modes. In general, switching between the two modes is determined depending on driver requested power/torque, vehicle speed, a battery state of charge (SOC), and the like and a switching condition to the HEV mode from the EV mode and a switching condition to the EV mode from the HEV mode are differently set. This is because, if the switching conditions between the two modes are set to be the same, the two modes are excessively frequently switched depending on change in driver requested power/torque, which will be described with reference to FIG. 1.
FIG. 1 is a diagram showing an example of a mode switching condition of a general hybrid vehicle.
In FIG. 1, switching between EV and HEV modes is assumed to be determined based on a driver requested power. In addition, in a graph of FIG. 1, a vertical axis indicates the driver requested power and a horizontal axis indicates time.
Referring to FIG. 1, when the driver requested power equal to or greater than an HEV mode entrance reference line (On Line) in the EV mode, the EV mode may be switched to the HEV mode. In addition, when the driver requested power is equal to or less than an EV mode entrance reference line (Off Line) in the HEV mode, the HEV mode may be switched to the EV mode. As a result, when the driver requested power exceeds On Line, a current mode is switched to the HEV mode but, even if the driver requested power is lowered below On Line, the HEV mode is maintained as long as the driver requested power above Off Line, thereby preventing modes from being frequently switched.
As such, the On Line and the Off Line are differently set, and in general, the Off Line is determined by subtracting predetermined hysteresis power (Hys.Power), i.e., Delta Power from On Line. Here, the On Line and the Hys.Power may be determined depending on vehicle speed and battery SOC.
Hereinafter, a problem that arises when a general mode switching condition is fixed will be described with reference to FIGS. 2 and 3.
FIGS. 2 and 3 are diagrams for explanation of a problem of a general mode switching condition.
First, when a mode switching condition is set to be fixed, it is not possible to control a time point of re-switching to an EV mode according to change in a travelling situation. For example, when EV driving is maintained due to a high battery SOC, fuel consumption may be prevented. To this end, to impede entrance into an HEV mode, On Line needs to be raised and, to guide switching to an EV mode from an HEV mode, hysteresis power needs to be lowered. As a result, for effective mode switching when a SOC is high, the mode switching condition needs to be the same as in FIG. 2. However, when the SOC is not high, appropriate hysteresis power cannot be ensured.
As shown in FIG. 3, even if hysteresis power is fixed, when On Line is set to be low, Off Line becomes lower than a coasting power line in a non-driving force state (e.g., coasting) in which a driver takes their foot off an accelerator pedal (APS off). That is, since the coasting power line that indicates requested power during coasting becomes driver requested power, Off Line is always lower than requested power and, accordingly, there is a problem in that an HEV mode is not capable of being released even in an accelerator pedal off (APS off) state.
Accordingly, there is a need for a mode switching control method for preventing modes from being frequently switched and preventing an inappropriate mode from being forcibly maintained.